1. Field of the Invention
The present invention relates to a magnetic field generating device used for an electron spin resonance device (hereinafter referred to as the "ESR Device") and more particularly to such type of magnetic field generating device that is capable of continuously varying a magnetic field between the magnetic poles of permanent magnets arranged opposite to each other leaving a desired space therebetween while the magnetic field is made to keep a high uniformity.
The ESR device is an instrument which is being made use of in the fields of archaeology and earth science and which is capable of measuring the ages of archaeological remains, geology, minerals and fossil remains.
That is, the ESR device can detect unpaired electrons due to natural radiation damages of the above-mentioned test objects and measure the ages of the objects from the detected values.
For the magnetic field generating device forming an essential part of the ESR device, it is necessary to form a magnetic field of about 3400 gauss in the space in which a test object is arranged and to continuously change the intensity of the magnetic field within the range of .+-. about 100 gauss.
2. Description of the Prior Art
As a magnetic field generating source for a magnetic field generating device used with the ESR device, there has hitherto been known a structure comprising electromagnets or a combination of an electromagnet and a permanent magnet.
It is easy to continuously change a magnetic field in a desired space by continuously varying a voltage applied on the electromagnets, but the intensity of the magnetic field is hardly adjusted in a stabilized state. Further, due to the necessity of providing an exciting power source, the entire device becomes inevitably large-sized and expensive.
Further, there has been proposed a structure using only permanent magnets as a magnetic field generating source so that the magnetic field intensity is changed by changing the opposing distance (length of the space) between the permanent magnets.
However, this structure has also had the disadvantage that when the distance between the opposing permanent magnets is changed, the uniformity of the magnetic field is impaired due to various causes including the deviation of the axis of one of the opposing permanent magnets from that of the other.
Usually, the uniformity of the magnetic field required of such magnetic field generating device is less than 1% and especially when an accurate measurement is required, a uniformity of less than 0.01% is considered necessary.